Conventional linearly polarizing elements, such as linearly polarizing lenses for sunglasses and linearly polarizing filters, are typically formed from stretched polymer sheets containing a dichroic material, such as a dichroic dye. Consequently, conventional linearly polarizing elements are static elements having a single, linearly polarizing state. Accordingly, when a conventional linearly polarizing element is exposed to either randomly polarized radiation or reflected radiation of the appropriate wavelength, some percentage of the radiation transmitted through the element will be linearly polarized.
In addition, conventional linearly polarizing elements are typically tinted. Typically, conventional linearly polarizing elements contain a coloring agent and have an absorption spectrum that does not vary in response to actinic radiation. The color of the conventional linearly polarizing element will depend upon the coloring agent used to form the element, and most commonly, is a neutral color (for example, brown or gray). Thus, while conventional linearly polarizing elements are useful in reducing reflected light glare, because of their tint, they are typically not well suited for use under low-light conditions. Further, because conventional linearly polarizing elements have only a single, tinted linearly polarizing state, they are limited in their ability to store or display information.
Conventional linearly polarizing elements are typically formed using sheets of stretched polymer films containing a dichroic material. Correspondingly, while dichroic materials are capable of preferentially absorbing one of two orthogonal plane polarized components of transmitted radiation, if the molecules of the dichroic material are not suitably positioned or arranged, no net linear polarization of transmitted radiation will be achieved. Without intending to be bound by any theory it is believed that due to the random positioning of the molecules of the dichroic material, selective absorption by the individual molecules will cancel each other such that no net or overall linear polarizing effect is achieved. As such, it is typically necessary to position or arrange the molecules of the dichroic material by alignment with another material so as to achieve a net linear polarization.
A common method of aligning the molecules of a dichroic dye involves heating a sheet or layer of polyvinyl alcohol (“PVA”) to soften the PVA and then stretching the sheet to orient the PVA polymer chains. Thereafter, the dichroic dye is impregnated into the stretched sheet, and the impregnated dye molecules adopt the orientation of the polymer chains. Resultantly, at least some of the dye molecules become aligned, such that the long axis of each aligned dye molecule is generally parallel to the oriented polymer chains. Alternatively, the dichroic dye can be first impregnated into the PVA sheet, and thereafter the sheet can be heated and stretched as described above to orient the PVA polymer chains and associated dye. In this manner, the molecules of the dichroic dye can be suitably positioned or arranged amongst the oriented polymer chains of the PVA sheet, and a net linear polarization can be correspondingly achieved. As a result, the PVA sheet can be made to linearly polarize transmitted radiation, and correspondingly a linearly polarizing filter can thus be formed.
In contrast to the dichroic elements discussed above, conventional photochromic elements, such as photochromic lenses that are formed using conventional thermally reversible photochromic materials are generally capable of converting from a first state, for example a “clear state,” to a second state, for example a “colored state,” in response to actinic radiation, and reverting back to the first state in response to thermal energy. Thus, conventional photochromic elements are generally well suited for use in both low-light and bright conditions. Conventional photochromic elements, however, that do not include linearly polarizing filters are generally not capable of linearly polarizing radiation. The absorption ratio of conventional photochromic elements, in either state, is generally less than two. Therefore, conventional photochromic elements are not capable of reducing reflected light glare to the same extent as conventional linearly polarizing elements. In addition, conventional photochromic elements have a limited ability to store or display information.
Photochromic-dichroic compounds and materials have been developed that provide both photochromic properties and dichroic properties, if properly and at least sufficiently aligned. When in a colored or darkened state, such as when exposed to actinic light, photochromic-dichroic compounds, however, typically have a larger percent transmittance than non-polarizing or conventional photochromic compounds at equivalent concentrations and sample thickness. While not intending to be bound by any theory, and based on the evidence at hand, it is believed that the increased percent transmittance of photochromic-dichroic materials in the darkened or colored state is due to the percent transmittance being an average of the two orthogonal plane polarized components of the polarized radiation. A photochromic-dichroic material will more strongly absorb one of the two orthogonal plane polarized components of the incident random radiation, resulting in one of the planes of transmitted polarized light (passing through and out of the sample) having a greater percent transmittance than the other orthogonal plane polarized component. The average of the two orthogonal plane polarized components typically results in an average percent transmittance of greater magnitude. In general, as the linearly polarizing efficiency, which can be quantified in terms of absorption ratio, of photochromic-dichroic compounds increases, the percent transmittance associated therewith also increases.
It would be desirable to develop new polarizing photochromic articles that include photochromic-dichroic compounds, and which provide a combination of linear polarizing properties, and reduced percent transmittance when in a colored or darkened state, such as when exposed to actinic light.